The invention concerns a method for synchronization of systems for seismic surveys and of the systems' subsystems, wherein the systems may be land-based or marine base or combinations of these and the subsystems are composed of, e.g., at least one navigation system, at least one seismic signal generating system and one or more recording systems.
When conducting seismic surveys, whether on land or at sea, it is vitally important to be able to determine the exact times of events and operations and also to be able to establish the exact time relationship between the different operations. In seismic surveys, however, there are a number of potential sources of timing error and these error sources will in turn affect the accuracy of the entire survey system. The use of modern computer technology in the processing of the seismic data has led to the need for an improvement of the accuracy in the time domain. It is a requirement that this accuracy should at least be substantially better than the accuracy and resolution which characterizes the software used and which normally is in the microsecond range. This will also make it possible to solve problems concerning the synchronization of the subsystems in a seismic survey system or the synchronization of several survey systems cooperating in a seismic survey.
According to prior art the synchronization in seismic surveys is based on the fact that the navigation system knows the time, vessel speed and position to an accuracy of approximately 100 ms in the time domain. The navigation system estimates the point in time when the ship arrives at the position for firing the next shot, and when this point in time is reached the navigation system generates an initiating pulse. This pulse is detected by a subsystem for the data recording which in turn generates a shot pulse to the source subsystem, which may be an airgun system. After a certain delay which corresponds to the source subsystem's response time, the shot is fired. At this point in time the source subsystem again generates a pulse which is detected by the recording subsystem, viz. the so-called "field-time-break" (FTB) pulse or time-mark signal. The recording subsystem then begins to sample the signals from the sensors, e.g., hydrophones. The sampling process is normally driver by a free-running oscillator. The signals are usually sampled every other microsecond in a period which normally lasts from 6 s to 8 s. If more than one ship is used in a survey, the initiating pulse is transmitted via a radio modem line to the ship or ships participating in the survey. This radio modem normally uses an audio channel with the delays and inaccuracies inherent in this type of connection. If simultaneous surveys are conducted at widely scattered locations, a broadcast time signal is used to time-mark the data, i.e. the data are not initially synchronized, but marked in such a way that the lack of synchronization can be compensated for during the data processing.
This known method therefore has a number of inherent weaknesses with regard to the synchronization of events, and operations and errors which occur at a point in time or in a subsystem will be transmitted through the system in the subsequent operations with the result that the total system in the time domain will be encumbered with errors which may be difficult to quantify. A generally known technique has been to use standardised time code signals in order, e.g., to synchronize processors in data recording systems. These time code signals may be based on known time code standards, e.g. IRIG (Inter-Range Instrumentation Group) A, B or G or NASA 36. There has, however, been no known method for satisfactorily implementing the use of time codes in seismic survey systems. Furthermore the time code has to be referred to an absolute time standard, e.g., universal time coordinated (UTC). Such an absolute time standard can be obtained to-day via a satellite navigation system, e.g. the global positioning system, (GPS) and, for example, supplied to the navigation system on board a vessel.